Carbon has traditionally been the most common material of choice for polymer electrolyte fuel cell (PEFC) electrocatalyst supports due to its low cost, high abundance, high electronic conductivity, and high Brunauer, Emmett, and Teller (BET) surface area, which permits good dispersion of platinum (Pt) active catalyst particles. However, the instability of the carbon-supported platinum electrocatalyst is a key issue that currently precludes widespread commercialization of PEFCs for automotive applications.
Carbon is known to undergo electrochemical oxidation to carbon dioxide. Despite the fact that the cathode potential is usually significantly higher than the standard potential for carbon oxidation, the actual rate of carbon oxidation is very slow due to a very low standard heterogeneous rate constant. During operation of automotive PEFC stacks, fuel/air mixed fronts are known to occur during stack startup and shutdown. Air usually fills the flow channels when the stack is nonoperational. During startup, the hydrogen fed into the stack displaces the air from the anode flow channels, leading to a mixed fuel-oxidant. These mixed-reactant fronts result in significant electrode polarization. Under these conditions, the PEFC cathode can experience high potentials, corresponding to a significantly higher overpotential for the carbon oxidation reaction. The electrochemical reaction rate constant, which increases exponentially with overpotential, is significantly enhanced during this period. Under these conditions, carbon corrosion is exacerbated.
In a second mechanism, fuel starvation at the anode catalyst sites as a consequence of fuel overutilization or flooding (lack of fuel access to catalyst site) also exacerbates carbon corrosion. In this case, carbon is oxidized to provide protons and electrons in place of the absent fuel.
The adverse consequences of carbon corrosion include (i) platinum nanoparticle agglomeration/detachment; (ii) macroscopic electrode thinning/loss of porosity in the electrode; and (iii) enhanced hydrophilicity of the remaining support surface. The first results in loss of catalyst active surface area and lower mass activity resulting from reduced platinum utilization, whereas the second and third result in a lower capacity to hold water and enhanced flooding, leading to severe condensed-phase mass transport limitations. Clearly, both consequences directly impact PEFC cost and performance, especially in the context of automotive stacks.